The invention is applicable particularly advantageously in cases in which the area to be observed, illuminated by the illumination, exhibits a very low reflectivity; as a result, the intensity, of the reflections of the illumination at optical boundary layers of the system, exceeds the utility light intensity or significantly interferes with the latter. Such systems include, for example, the ophthalmoscopic lens of a fundus camera used for the mapping of the eyeground since the degree of reflection of the fundus to be observed is very low.
According to known prior art, the basic design of a fundus camera exhibits a multilevel optical system. Thereby, an intermediate image of the retina is produced by an ophthalmoscopic lens, which is then mapped by an optical tracking system in an additional intermediate image and/or onto a film or photooptical array in the form of a CCD matrix. Thereby, the ophthalmoscopic lens is part of the illumination system as well as the imaging system.
A particular problem with fundus observation and imaging are the reflections at the cornea and the surfaces of the ophthalmoscopic lens since the intensity of the light, which is reflected by the retina and contains the actual image information of interest, is significantly lower than the intensity of the light, which is reflected before it enters the eye.
Disruptive cornea reflections are usually prevented through a separation of the pupil of the eye. Thereby, the ophthalmoscopic lens projects an illumination ring in the eye pupil, whereby the beams of the illumination, reflected on the cornea, miss the aperture of the observation system. Therefore, only the area within the illumination ring is used for observation. For the suppression of the reflections from the ophthalmoscopic lens, three concepts are essentially known, according to prior art.
DE 35 19 442 A1 describes an optical system in which light components which could enter the observation aperture through the reflection at the ophthalmoscopic lens or at the cornea are blocked out by means of black point plates. Thereto, the black point plates are arranged in a defined manner at a suitable location in the beam path and coated with light-absorbing layers. This type of reflection suppression has come to be known as “anti-reflection point.”
A disadvantage of this concept is the proximity of the anti-reflection point to the field stop. The absorption of the individual light components can become visible as irregular illumination of the fundus. Ring-shaped shadows occur which impair the image impression and, therefore, impede the evaluation by the eye doctor.
Another solution is described in DE 103 16 416 A1. Thereby, the blocking of certain light components within the illumination optics is foregone. In place of the ophthalmoscopic lens, a multilens objective is provided, the lenses of which are tilted relative to one another in such a way that direct reflections at the optical boundary layers do not enter the aperture of the observation system.
Said solution requires significant expenditures for the mechanical mountings since a classical mounting of rotationally symmetric systems is not applicable. In order to keep the optical elements small, wedge-shaped lens segments develop which require a special mounting technology which has a negative impact on this second concept.
The use of lenses with only positive refractive power causes longitudinal chromatic aberrations and transverse chromatic aberrations which have to be elaborately compensated in the observation portion as well as the illumination portion of the downstream optical system. For applications with very small beam diameters, such as laser applications, the high number of optical boundary layers and the long glass path of the described objective also have a negative impact. Even slight contamination at the boundary layers and in the material can have adverse effects. As a result, the intensity greatly decreases and interfering stray light occurs.
The solution described in U.S. Pat. No. 4,730,910 A relates to an optical imaging system consisting of wide-angle lenses with stray light deflection, whereby illumination and imaging beam path are separated from one another. However, for the stray light deflection, the individual lenses are shifted so far that only one half of the lenses are utilized for the beam path.
This solution is disadvantageous in that several lenses are required for an effective correction of the resulting image defects and distortions. This significantly increases the expenditures regarding mounting and alignment.
Further focusing, particularly reflection-deflecting, spherical lens arrangements are described in U.S. Pat. No. 4,415,239 A. Thereto, the lens arrangements consist of a series of at least two optical elements with a cylindrical component each.
In order to block out the resulting reflections in said arrangements, it is necessary to significantly tilt the optical elements, which negatively impacts the image quality. Even though it is hereby also possible to reduce the extent of the tilting through a toric effect of the optical elements, the resulting higher-order aberrations, however, are unavoidable with this design. In addition, said approach to a solution also requires several lenses for an effective correction of the resulting image defects and distortions, significantly increasing the expenditures regarding mounting and alignment.
The third known concept, according to prior art, provides for the use of at least one mirror element instead of the ophthalmoscopic lens. Such systems exhibit simple mirror geometries, with which only a small observation field and/or illumination field can be realized with sufficient optical quality.
Other mirror systems, as described, e.g., in U.S. Pat. No. 6,585,374 B2, utilize movable parts in order to expand the small observation field and/or illumination field through scan movements. Thereto, elaborate mechanics for the precise movement and elaborate image processing techniques are required.
A further solution to said third concept is described in WO 2008/077526 A2. Through the use of mirror elements instead of the ophthalmoscopic lens, interfering reflections and chromatic aberrations of the optical media, caused by dispersion, can be avoided.
The optical system for ophthalmological devices and, particularly, fundus cameras, described in DE102008026576 A1, also relates to said third concept. With this solution it is possible to realize images of the eye fundus of very high quality, however, the manufacture of the at least two reflecting optical elements as well as mounting and alignment are elaborate and difficult.